Approximately fifty-five percent of all tractor semi-trailer combination vehicle operator fatalities occur in rollover accidents. One or more wheels lift off of the ground in the initial stages of vehicle rollover. Wheel lift, however, is almost imperceptible to the operator until the vehicle begins to roll. Unfortunately, once the operator perceives that the vehicle has tilted and is starting to rollover, it may be too late for the operator to prevent rollover.
Vehicles with a high center of gravity such as long haul trucks and truck semi-trailer combinations are particularly susceptible to rolling over during cornering at relatively moderate speeds. FIG. 1 illustrates the physical forces that act on a vehicle 10 to cause it to rollover. The vehicle has a center of gravity (cg), and the height of the center of gravity (hcg) is the distance between this point and the ground. During steady cornering, lateral or sideways acceleration occurs, and the vehicle is influenced by a downwards force (mg) due to gravity and a lateral force (maLAT) due to lateral acceleration. When the vehicle is at rest or traveling in a straight line, the downwards force is equally distributed between the wheels at each axle as wheel load, that equals the normal force on each wheel (FN1) (FN2). During cornering, however, lateral acceleration causes a sidewise imbalance between the inner wheels 12 and the outer wheels 14 due to forces (FN1) and (FN2) that change the wheel load at each axle. Although the sideway imbalance force FN depends on several parameters such as torsional stiffness and curve radius, the these parameters may be value may be approximated a constant C and the sideways imbalance force FN may be calculated with the following equation:FN=C*hcg*aLAT As the lateral acceleration increases, the sideway imbalance force FN reduces the downwards wheel load on the inner wheels, and increases the downwards wheel load on the outer wheels. If the lateral acceleration exceeds a safe level, the inner wheel load is reduced to zero the vehicle rolls over.
FIG. 2 is a graph that illustrates the inverse relationship between the height of the center of gravity and the lateral acceleration value as vehicle rollover is approached. The solid line represents a vehicle with a high center of gravity, and the dashed line represents a vehicle with a low center of gravity. The vehicle overturns when the inner wheel load equals zero. As shown in the graph, almost twice as much lateral acceleration is required for the vehicle with the low center of gravity to overturn as is required for the vehicle with the high center of gravity.
In the past, basic rollover sensors have been employed in vehicles that detect an impending rollover condition by physically measuring the angular position of the vehicle. These basic rollover sensors use a pendulum that normally hangs vertically downward due to the earth's gravitational force. Many basic vehicular sensing devices simply monitor the angular position of the vehicle relative to a level ground horizontal position. Thus, the basic vehicle rollover sensors are susceptible to error when the vehicle travels around a turn or becomes airborne, because the earth's gravitational force, which the sensor relies on, may be overcome by other forces.
More recently, sophisticated rollover sensing systems have been utilized. One such approach requires the use of a plurality of sensors including accelerometers and angular rate sensors, also referred to as gyros, all of which are employed together for use in an inertial navigation system which tracks position and attitude of the vehicle. The accelerometers generally provide lateral, longitudinal, and vertical acceleration measurements of the vehicle, while the gyros measure pitch rate, roll rate, and yaw rate. However, the more sophisticated rollover sensing approaches generally require several costly high-precision sensors. In addition, these systems are susceptible to cumulative drift errors, and therefore they must be re-calibrated when needed.
It is therefore an object of the present invention to provide a vehicle rollover prediction and prevention system that requires a minimum of sensors and is relatively immune to errors generally found in conventional automotive-grade sensors. It is another object of the present invention to provide for vehicle rollover sensing for a vehicle that may predict a rollover condition and allow time to deploy automatic braking or engine velocity reduction. It is a further object of the present invention to provide for reliable vehicle rollover prediction and prevention with a relatively simple low-cost device.